At a Chicago convention jammed with thousands of doctors in 2015, a buzz of excitement rippled through the hall as a researcher revealed data showing that a Bristol-Myers Squibb Co. drug cocktail had remarkable success in fighting a deadly form of skin cancer.

It was a high point in the development of a promising class of cancer treatments known as immunotherapy that have the potential to radically change the course of the disease and generate billions of dollars in sales. And Bristol-Myers stood at the forefront with its drug, Opdivo, with high hopes of dominating the market by treating other forms of cancer as well.

That euphoria has worn off.

In the past year, Bristol-Myers has seen $37 billion in market value wiped out after it gambled -- and lost -- on a clinical trial of Opdivo that could have made it a treatment in the lucrative market for new lung cancer patients. Its top rival, Merck & Co., is now viewed by investors as the favorite in that race. This week, coming out of the same cancer-research meeting that seemed so promising two years ago, Bristol-Myers shares have continued to slide as talk swirls about a possible sale.

“This is a blisteringly fast race for patients,” said Seamus Fernandez, an analyst at Leerink Partners, who rates Bristol-Myers stock “outperform.” He called the company’s decision on lung cancer “a major misstep.”

At least one prominent investor wants Bristol-Myers to expand its immune-therapy portfolio as much as possible or consider a merger with another drugmaker to accomplish that, according to a person with knowledge of the shareholder’s thinking. Analysts have mentioned Pfizer Inc. as potential suitor, along with Novartis AG and Gilead Sciences Inc.

Opdivo Optimism

Bristol-Myers is urging patience as it awaits scientific data on a host of combination therapies -- including mixing Opdivo with other medicines. That would expand the market for Opdivo, the company’s best-selling drug and a key to its oncology program. Cancer has become increasingly important to Bristol-Myers, which shed its diabetes unit in 2013. The company’s website lists just one new drug in a late-stage trial -- for cancer.

“We are addressing cancer from all angles and our deep understanding of cancer biology and industry-leading pipeline will enable us to identify the right treatment, for the right patient, at the right time,” said Audrey Abernathy, a spokeswoman.

Bristol-Myers’s cancer business had fallen into a dry spell before the company went on a deal-making spree over the past decade. Its 2009 purchase of Medarex, which was developing Opdivo, changed that. The drug, which harnesses the body’s own immune system to attack tumors, now accounts for almost 20 percent of Bristol-Myers’s $19.4 billion in revenue.

Opdivo’s success in treating melanoma had investors hoping that the company would have similar results in lung cancer, a field with vastly more patients. Other rivals -- including Merck -- saw the same potential, and were developing similar drugs.

Facing Choices

As New York-based Bristol-Myers geared up for a race, it had a choice. It could either go for a wide group of lung cancer patients who had a low level of a tumor protein that indicated the drug might help them. Or it could build a trial around a narrower group -- and smaller market -- of patients with high concentrations of the biological signal.

Some of Bristol-Myers’s external scientific advisers raised concerns about the broad approach, according to two people familiar with the matter. The company went ahead anyway. Bristol-Myers didn’t respond directly to questions about its internal discussions.

It also decided to have the trial measure whether its treatment stopped patients’ tumors from getting worse, instead of whether the drug actually helped them live longer -- the gold standard in cancer research. That was in hopes of showing positive results sooner than Merck, one of the people said.

In August, Bristol-Myers was stung when the trial failed to pave the way for Opdivo to be used on its own for many more lung cancer patients. The news sent its shares plummeting 20 percent over two days. Five months later, the drugmaker said it wouldn’t seek accelerated U.S. Food and Drug Administration approval for paring Opdivo with another drug, Yervoy, a sign that the trial results for that combination may also be underwhelming.

Merck Boon

Meanwhile, Merck’s Keytruda, a similar drug, is now being tested in more than half of the 1,000 imunotherapy trials underway. It’s on track to outsell Opdivo in 2018, according to analysts’ estimates compiled by Bloomberg. That’s a reversal from last year, when Keytruda’s sales were less than half of Opdivo’s.

Merck benefited by taking a narrower approach than Bristol-Myers. It studied its drugs in a smaller population of cancer patients, those who had biomarkers indicating that the treatment was more likely to succeed.

“Merck has clearly won the battle in lung cancer,” said Alex Spira, an oncologist at the Virginia Cancer Institute who overees the institute’s early-stage trial program.

Merck declined to comment.

Bristol-Myers Chief Science Officer Thomas Lynch, who served as a board director until he was tapped to lead research and development in March, says the company remains committed to developing first-line treatments for lung cancer, which will come from combinations of different drugs.

Lynch wants to strike more partnerships while emphasizing biomarkers, and take a disciplined approach to trial design. He compared the company’s position to that of the New England Patriots football team, which overcame a 25-point deficit to win the Super Bowl in February.

“We are early in the game,” said Lynch, an oncologist who who spent more than two decades at Massachusetts General Hospital. “It is just the first quarter.”

Activist Action

Investors might not be so patient. In February, Bristol-Myers added three new directors and agreed to buy back $2 billion of its shares after holding discussions with the hedge fund Jana Partners LLC. The activist investor Carl Icahn also purchased a stake, according to the Wall Street Journal, fueling speculation about a possible sale.

A spokesman for Jana, which has since reduced its stake, declined to comment. Icahn couldn’t be reached for comment.

Merck is only one competitor Bristol-Myers needs to worry about. Roche Holding AG and AstraZeneca Plc are also in the race to develop treatments. Now Wall Street is waiting to see the results of Bristol-Myers’s combination trial to understand how well the cocktail works in lung cancer patients.

“If that trial’s not successful, or it’s equivalent, what’s next?” said John Schroer, sector head of health care at Allianz Global Investors, which own Bristol-Myers shares. “That’s a conversation I don’t think a lot of investors want to have.”